Modern wireless communication networks achieve high data rates by adopting advanced modulation schemes and employing multiple-input-multiple-output (MIMO) antenna systems. While such technologies are successful in achieving high spectrum efficiencies, they pose new challenges to multi-antenna transmitter design.
For example, high order modulation schemes, such as 16 QAM modulation or 64 QAM modulation, result in a transmit signal with a high Peak-to-Average Power Ratio (PAPR). Orthogonal frequency-division multiplexing (OFDM) techniques used by MIMO antenna systems also result in a signal with a high PAPR. One consequence of a high PAPR is that the power amplifier (PA) must meet stricter linearity requirements to avoid spectrum re-growth, especially when the transmit power is high. Power amplifiers typically operate most efficiently at or near the saturation point. However, the response of the power amplifier at or near saturation point is non-linear. Therefore, there is a trade-off between greater efficiency and linearity.
MIMO systems employ multiple antennas at the transmitter and receiver to transmit and receive information. By exploiting the spatial dimension of the communication channel between the transmitting terminal and the receiving terminal, MIMO communication systems can simultaneously transmit multiple data streams from a transmitting terminal to one or more receiving terminals over the same carrier frequency. Thus, MIMO communication systems achieve higher spectral efficiency and higher data rates without increasing bandwidth. One problem encountered in MIMO transmitters is crosstalk between the transmit paths, especially when multiple transmit paths are integrated in a limited area on the same chipset. Crosstalk affects transmit signal quality, which can be measured by adjacent channel power ratio or error vector magnitude. Crosstalk also affects received signal quality.
One way to improve a power amplifier's efficiency and its overall linearity is to digitally predistort the input signal to the power amplifier to compensate for the distortion introduced by the power amplifier. In effect, the input signal is adjusted in anticipation of the distortion to be introduced by the power amplifier, so that the amplifier output signal is largely free of distortion. Generally, the predistortion is applied to the signal digitally before the signal is up-converted to radio frequencies.
Some known techniques are also useful in reducing crosstalk. They include buffering local oscillator paths, installing grounded guard rings, and using a deep trench, a porous silicon trench, a silicon-on-insulator substrate, or a high-resistivity substrate obtained by proton bombardment. However, these techniques do not eliminate crosstalk completely.
Article Crossover Digital Predistorter for the Compensation of Crosstalk and Nonlinearity in MIMO Transmitters, IEEE Transactions on Microwave Theory and Techniques, Vol. 57, No. 5, May 2009, describes a digital crossover predistorter that uses a combined mathematical model for both crosstalk and power amplifier distortions. Such crossover predistorters can be beneficial in improving the overall performance of a transmitter system in terms of both linearity and efficiency. However, the structure of the crossover predistorters can become complex as the number of transmit paths increases. The crossover predistorters become impractical in situations where the number of transmit paths becomes large because the complexity of the digital predistorter increases quadratically with the number of RF transmit paths. In a MIMO transmitter that comprises 4 or 8 RF paths, a crossover predistorter that models both crosstalk and PA distortions as proposed in the above-mentioned article is 8 or 32 times more complex than modeling PA distortions alone.